By Alton Parrish (Reporter)
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This diagram illustrates Mars’ “thermal tides,” a weather phenomenon responsible for large, daily variations in pressure at the Martian surface. Sunlight heats the surface and atmosphere on the day side of the planet, causing air to expand upwards. At higher levels in the atmosphere, this bulge of air then expands outward, to the sides, in order to equalize the pressure around it, as shown by the red arrows. Air flows out of the bulge, lowering the pressure of air felt at the surface below the bulge. The result is a deeper atmosphere, but one that is less dense and has a lower pressure at the surface, than that on the night side of the planet. As Mars rotates beneath the sun, this bulge moves across the planet each day, from east to west. A fixed observer, such as NASA’s Curiosity rover, measures a decrease in pressure during the day, followed by an increase in pressure at night. The precise timing of the increase and decrease are affected by the time it takes the atmosphere to respond to the sunlight, as well as a number of other factors including the shape of the planet’s surface and the amount of dust in the atmosphere.

Image credit: NASA/JPL-Caltech/Ashima Research/SWRI

During the first 12 weeks after Curiosity landed in an area named Gale Crater, an international team of researchers analyzed data from more than 20 atmospheric events with at least one characteristic of a whirlwind recorded by the Rover Environmental Monitoring Station (REMS) instrument. Those characteristics can include a brief dip in air pressure, a change in wind direction, a change in wind speed, a rise in air temperature or a dip in ultraviolet light reaching the rover. Two of the events included all five characteristics.

In many regions of Mars, dust-devil tracks and shadows have been seen from orbit, but those visual clues have not been seen in Gale Crater. One possibility is that vortex whirlwinds arise at Gale without lifting as much dust as they do elsewhere.

NASA’s Mars rover Curiosity used a mechanism on its robotic arm to dig up five scoopfuls of material from a patch of dusty sand called “Rocknest,” producing the five bite-mark pits visible in this image from the rover’s left Navigation Camera (Navcam). 

Image credit: NASA/JPL-Caltech

“Dust in the atmosphere has a major role in shaping the climate on Mars,” said Manuel de la Torre Juarez of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif. He is the investigation scientist for REMS, which Spain provided for the mission. “The dust lifted by dust devils and dust storms warms the atmosphere.”

Dominant wind direction identified by REMS has surprised some researchers who expected slope effects to produce north-south winds. The rover is just north of a mountain called Mount Sharp. If air movement up and down the mountain’s slope governed wind direction, dominant winds generally would be north-south. However, east-west winds appear to predominate. The rim of Gale Crater may be a factor.

“With the crater rim slope to the north and Mount Sharp to the south, we may be seeing more of the wind blowing along the depression in between the two slopes, rather than up and down the slope of Mount Sharp,” said Claire Newman, a REMS investigator at Ashima Research in Pasadena. “If we don’t see a change in wind patterns as Curiosity heads up the slope of Mount Sharp — that would be a surprise.”

REMS monitoring of air pressure has tracked both a seasonal increase and a daily rhythm. Neither was unexpected, but the details improve understanding of atmospheric cycles on present-day Mars, which helps with estimating how the cycles may have operated in the past.

The seasonal increase results from tons of carbon dioxide, which had been frozen into a southern winter ice cap, returning into the atmosphere as southern spring turns to summer. The daily cycle of higher pressure in the morning and lower pressure in the evening results from daytime heating of the atmosphere by the sun. As morning works its way westward around the planet, so does a wave of heat-expanded atmosphere, known as a thermal tide.

Effects of that atmospheric tide show up in data from Curiosity’s Radiation Assessment Detector (RAD). This instrument monitors high-energy radiation considered to be a health risk to astronauts and a factor in whether microbes could survive on Mars’ surface.

“We see a definite pattern related to the daily thermal tides of the atmosphere,” said RAD principal investigator Don Hassler of the Southwest Research Institute’s Boulder, Colo., branch. “The atmosphere provides a level of shielding, and so charged-particle radiation is less when the atmosphere is thicker. Overall, Mars’ atmosphere reduces the radiation dose compared to what we saw during the flight to Mars.” 

Twenty-one times during the first 12 weeks that NASA’s Mars rover Curiosity worked on Mars, the rover’s Rover Environmental Monitoring Station (REMS) detected brief dips in air pressure that could be caused by a passing whirlwind. The blue line in this chart shows two examples, both shortly after 11 a.m. local Mars time, when the air pressure dipped on the 75th Martian day, or sol, of the mission (Oct. 25, 2012). In both cases, wind direction monitored by REMS changed within a few seconds of the dip in pressure, as indicated by the green line on the chart. That is additional evidence that the pressure dips were whirlwinds.

Signs of a Whirlwind in Gale Crater

A Finnish, Spanish and American team is using REMS, which Spain provided for Curiosity, to watch for signs of dust devils — whirlwinds carrying dust. 

 

In many regions of Mars, dust-devil tracks and shadows have been photographed from orbit, but those visual clues have not been seen at Gale Crater, where Curiosity is working. The evidence from REMS indicates that whirlwinds may be forming in Gale Crater. While Curiosity is watching for them with cameras on some days, researchers are also considering the possibility that these swirling, convective winds do not lift as much dust at Gale as in other parts of Mars.

In this chart, the air-pressure scale is in Pascals. The wind direction scale is an estimate in degrees relative to the front of the rover. On Sol 75, the rover was facing approximately westward, and 90 degrees on this graph indicates winds coming from the north.

Image credit: NASA/JPL-Caltech/ CAB (CSIC-INTA)

The overall goal of NASA’s Mars Science Laboratory mission is to use 10 instruments on Curiosity to assess whether areas inside Gale Crater ever offered a habitable environment for microbes. 

Potential Sources and Sinks of Methane on Mars

If the atmosphere of Mars contains methane, various possibilities have been proposed for where the methane could come from and how it could disappear.

Potential non-biological sources for methane on Mars include comets, degradation of interplanetary dust particles by ultraviolet light, and interaction between water and rock. A potential biological source would be microbes, if microbes have ever lived on Mars. Potential sinks for removing methane from the atmosphere are photochemistry in the atmosphere and loss of methane to the surface.

Image credit: NASA/JPL-Caltech, SAM/GSFC

Rock ‘Burwash’ Near Curiosity, Sol 82

This focus-merge image from the Mars Hand Lens Imager (MAHLI) on the arm of NASA’s Mars rover Curiosity shows a rock called “Burwash.” The rock has a coating of dust on it. The coarser, visible grains are windblown sand.

The focus merge combines portions of eight images taken with the camera held in one position while the MAHLI focus mechanism moved for each of the eight exposures to capture features at different distances in focus. The images were taken during the mission’s 82nd sol, or Martian day (Oct. 29, 2012).

MAHLI viewed the rock from a distance of about 4.5 inches (11.5 centimeters). The image covers an area about 3 inches by 2.2 inches (7.6 centimeters by 5.7 centimeters). Burwash is located near the rover’s left-front wheel where the rover has been stationed while scooping soil at the site called “Rocknest.”

Image Credit: NASA/JPL-Caltech/MSSS

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